[jboss-jira] [JBoss JIRA] Commented: (JGRP-755) ProtocolStack: verify that TimeScheduler gets garbage collected on redeployment

[Bela Ban (JIRA)]

    [ http://jira.jboss.com/jira/browse/JGRP-755?page=comments#action_12412851 ] 
            
Bela Ban commented on JGRP-755:
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Thinking a bit more about this, I realized that a stopping channel removes all of its submitted tasks from the timer, so if we have 3 channels which hold refs to a singleton timer, and they are closed, the timer has no references to any threads anymore, so it should get GC'ed.

Oh, shit, not true: the TimeScheduler is a subclass of ScheduledThreadPoolExecutor, which maintains a pool of threads. Unless they are daemon threads, this will prevent the timer from being GC'ed. Do you agree on this ?

I can make the timer's threads daemon threads, which would allow timer to be GC'ed.


> ProtocolStack: verify that TimeScheduler gets garbage collected on redeployment
> -------------------------------------------------------------------------------
>
>                 Key: JGRP-755
>                 URL: http://jira.jboss.com/jira/browse/JGRP-755
>             Project: JGroups
>          Issue Type: Task
>            Reporter: Bela Ban
>         Assigned To: Bela Ban
>             Fix For: 2.7
>
>         Attachments: ThreadLeakTest.java
>
>
> ProtocolStack hold a static ref to TimeScheduler, making the timer a JVM singleton. However, what happens if the channel is redeployed and thus reloaded in a different classloader ? The timer would be created again, because static refs are *per classloader*. 
> The question is what happens to the old timers ? Are they GC'ed ?
> [Brian] 
> Following up from our chat yesterday re: static initialization of TimeScheduler.
> My memory re cleanup *was* off. I was thinking of some stuff in java.util.Timer; doesn't really apply.
> As you suspected, if you have a static ref to a TimeScheduler you'll leak the time scheduler and whatever classloader loaded JGroups (i.e. if a user deployed JGroups in a sar or something).
> This isn't exact, but it shows the logical reference chain:
> ProtocolStack.class.timeScheduler --> TimeScheduler.poolThreads[0] --> Thread.contextClassLoader --> ClassLoader.loadedClasses --> ProtocolStack.class.timeScheduler
> That looks circular, but the system holds a ref to the live Thread in the middle and that keeps the whole thing from being gc'd.
> Perhaps a way to deal with this is to maintain a static WeakReference<TimeScheduler> and then access it via a static method. Any ProtocolStack holds a strong ref to the scheduler, preventing it being gc'd, but once all channels are closed no ProtocolStack == available for gc.
> FYI, I've attached a little main() class that experiments with something analogous to holding a static ref to a TimeScheduler. You can see that with the static strong ref in place, the thread and its TCCL don't get gc'd but once only a static WeakReference is held, things get cleaned up.

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